Servo control of ink jet pump

ABSTRACT

An important factor in quality of printing with an ink jet printing apparatus is the velocity of the ink jet stream. The present case describes a number of servo systems for controlling velocity of the stream. This can be done indirectly by sensing pressure and/or temperature or directly by sensing velocity of the stream and controlling the pump frequency or pump drive currents.

United States Patent Fillmore et al.

[ Jan. 22, 1974 SERVO CONTROL OF INK JET PUMP [54] 3,610,782 10/1971 McGuire 417/326 [75] Inventors: Gary L. Fillmore; Hugh E. Naylor,

ggi g zgg west an of Primary Examiner-Joseph W. Hartary g Attorney, Agent, or Firm-D. Kendall Cooper [73] Assignee: International Business Machines Corporation, Armonk, NY.

22 F1 d: S t. 25, 1972 1 ep 57 ABSTRACT [2]] Appl. No.: 293,300

An important factor in quality of printing with an ink [52] US. Cl 346/75, 346/140, 417/32, jet printing apparatus is the velocity of the ink jet 417/43 stream. The present case describes a number of servo [51] Int. Cl. G01d 15/18 systems for controlling velocity of the stream. This can [58] Field of Search.. 346/75, 140; 417/412, 32, 42, be done indirectly by sensing pressure and/or temper- 417/43, 326; 318/127, 129, 130 ature or directly by sensing velocity of the stream and controlling the pump frequency or pump drive cur- [56] References Cited rents.

UNITED STATES PATENTS 3,296,624 H1967 Ascoli 346/140 13 Claims, 6 Drawing Figures 5 MACHINE {6 Q6 CLOCK RECOGNITION CR h. INTER CHARACTER; MACHINE HOME POSITION LOGIC I 5 I H REFERENCE j COMPARATOR 5 FSENSTRT" SENSOR 1 2 INK JET SYSTEM PAIENIED 3.787. 882

SHEET 1 RT 3 MACHINE 16 FIG. 1 6 CLOCK 4 RECOGNITION CR INTER- CHARACTER; MACHINE 17 MUME POSITION mm m PUMP 5 CONTROL REFERENCE COMPARATOR f5 4/ -AMP 7 T To sEM UR M l SENSOR 2 H 12 gpump INK JET SYSTEM 5 FIG. 2 I 2 TEMPERATURE 2s SENSOR j". H PRESSURE 25 SENSOR PP) INK TEMPERATURE OSCILLATOR SUPPLY REFERENCE E VOLTAGE as PATENTEU M2? 3974 3. 787. 882

SHEET 2 UF 3 44 45 FIG. 3a gws Q 46 N REFERE CE 2 7 COMPARATOR /50 COMPARATOR. REFERENCE 4 s2 so 61 m ifi 620 FIG. 3b

MACHINE GATE w I 600 Y CLOCK 61Q X 5 COUNTER RA GEN R DIGILTgb LEVEL ANA c n 67/ CONVERTER PUMP F72 CONTROL PUMP CONTROL FIG. 4 9o 91 AMP 92 T0 PUMP a2 1 85 L, 76 78 COMPARATOR CONTROL 5 AMP gfbizl -f'f j n \94 PATENTEBJANZEIHH SHEET 3 OF 3 FIG. 5

RELATED PATENT APPLICATION U. S. Pat. application Ser. No. 266,790 filed June 27, 1972, entitled Ink Jet Synchronization and Failure Detection System, and having James D. Hill, et al., as inventors.

BACKGROUND OF THE INVENTION AND PRIOR ART Various types of ink jet printing devices have been proposed heretofore. In one such system, drops of ink are formed and propelled from a nozzle toward a record medium, variably charged according to a signal representative of a wave form or character and deflected by deflection plates having a constant potential applied thereto. To insure good placement of drops in forming the waveform character, as the case may be, it is vital that the velocity of the ink drops remain in a predetermined range. Velocity of drops can vary considerably due to variations in temperature, pump pressure, and the like. The primary variation is due to temperature which causes large changes in the ink viscosity and hence the ink velocity as it leaves the nozzle. The present invention is intended to maintain velocity as constant as possible.

SUMMARY OF THE INVENTION A number of arrangements are described in the present case for controlling velocity of ink drops in an ink jet printer either directly or indirectly. In one case, the temperature and/or pressure of the ink is sensed at the pump and appropriate adjustments made to the pump driving circuit to increase or decrease pump pressure and thereby increase or decrease velocity of the stream. Another version contemplates the positioning of sensors adjacent the stream of drops for inducement of a voltage as charged drops pass by the sensors and for development of corrective signals to again control pump pressure and velocity of the stream. This version can be implemented in a digital or analog fashion, as desired. In another arrangement, sensors are positioned outside the normal range of drop deflection. During servo action, maximum deflection of the stream occurs for development of potentials to control the pump with corrective action, as necessary, to increase or decrease pump pressure, and thereby change velocity of the stream. The servo arrangements set forth make use of a highly efficient pump structure based on voice coil driving principles.

OBJECTS The primary object of the present invention, of course, is to sense various parameters in an ink jet printing system in order to develop corrective signals for controlling pump pressure and/or frequency to ultimately maintain velocity of the ink jet stream within a desired velocity range.

The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of the preferred embodiments of the invention as illustrated in the accompanying drawings.

DRAWINGS In the Drawings FIG. 1 is a system representation illustrating servo control of a pump in an ink jet system.

FIG. 2 illustrates velocity control by sensing of temperature and pressure.

FIG. 3a illustrates an arrangement for sensing velocity of an ink drop stream to develop digital count levels and conversion to analog for pump control, while FIG. 3b is essentially the same system wherein analog levels are developed directly.

FIG. 4 illustrates servo control involving the sensing of the maximum deflection of an ink jet stream.

FIG. 5 is a cross-sectional view of a voice-coil pump that is useful in the various embodiments of FIGS. 1-4.

DETAILED DESCRIPTION FIG. 1 is a generalized version representative of the various systems set forth in greater detail in FIGS. 2-4. An ink jet system 1, usually comprising an ink jet printing device, or the like, has an associated sensor 2, such as a temperature sensor. The ink jet system may be of the type set forth in the Hill, et al., patent application referred to previously. An output developed by sensor 2 is provided to an amplifier circuit 3 and from there to a comparator circuit 4. Another input to comparator circuit 4 is a reference signal on line 5. The output of comparator 4 is applied to pump control circuit 7 and is used to develop a control signal by lines 10 and 11 to coil 12 of pump 13. Servo action may be provided under ordinary circumstances from machine clock 16 through machine logic 17 in conjunction with recognition block 6. Thus the operation of the servo system shown in FIG. 1 would usually take place during nonprinting intervals such as during a carrier return (CR) interval, or home position between printing of characters that is, inter-character intervals, and the like, as recognized by block 6. As an alternative and as will be described shortly, a pressure sensor 20 associated with pump 13 provides inputs to amplifier circuit 3 instead of sensor 2. If pressure is sensed by sensor 20, as an example, provision is made to develop a corrective signal from pump control circuit 7 in order to increase or decrease pressure of pump 13 from the input signals related to pressure in pump 13. If the velocity of the stream is too slow, indicated by a low pump pressure, it may be increased by increasing the voltage applied to coil 12 of pump 13. Further, the frequency of signal applied to coil 12 may be changed to change pump pressure.

This is further illustrated in FIG. 2 where pump 13 is shown with associated coil 12. The pump assembly is further associated with nozzle 22 emitting a stream of ink drops 23 directed toward a record medium 25 for printing of characters or waveforms. In the event drops are not required for printing they are directed to a gutter 27. Ink is supplied through pump 13 to nozzle 22 from ink supply 29 by conduit 30. Pump 13 is a pump which is controlled by coil 12 such that the pressure is a function of coil current. Pressure sensor 32 monitors pump pressure and feeds a voltage analogous to pressure to amplifier 35. Amplifier 35 compares the pressure signal to the reference signal provided by amplifier 36 and adjusts a voltage controlled oscillator 38 so that current I to pump 13 minimizes the difference between the reference input and the sensor 32 output,

thus holding pressure constant. A manually set adjustment at oscillator 38 allows an initial factory pressure adjustment to be made. Amplifier 36 compares temperature reference voltage from block 40 with temperature sensor voltage from sensor 41 and adjusts the pressure reference voltage input for amplifier 35. This causes pressure to follow temperature change to hold velocity constant.

An ink jet system, without initial adjustments, could have as much as a to l variation in deflection sensitivity. This is due, in a large part, to variation in fluid flow through the nozzle. Adjusting pressure to obtain a constant velocity reduces this to a 5 to l variation. Adjusting for zero temperature effect could further reduce this variation to 1.5 or 2 to 1. At that point, including other system tolerances, an adjustment of deflection voltage would hold the machine to an acceptable level of performance.

By servo controlling pressure and automatically adjusting for temperature variation, electrical parameters are monitored rather than mechanical parameters. The system can easily compensate for different ink characteristics. Also, less precise tolerances are possible in the nozzle and in ink'batch to batch variation.

Instead of controlling coil current to adjust pressure, a constant current but variable frequency oscillator could be utilized to operate the pump. A wide variety of sensors can be used for pressure sensor 32 and temperature sensor 41. As an example, a thermocouple gauge may serve for temperature sensor 32.

In summary, the circuit of FIG. 2 holds pressure in an ink jet printer constant by means of a servo loop. lt further allows the reference pressure of the servo loop to be temperature compensated so that constant ink jet velocity is maintained with time and temperature variation. The servo loop eliminates the dependence upon relatively wide range and difficult to control mechanical tolerances and replaces them with more stable and easily controlled electrical tolerances.

If desired, either a pressure sensor or a temperature sensor could be used alone in conjunction with the pump for monitoring and changing pump pressure to thereby control velocity. The servo system of FIG. 2 could be set up to maintain a constant pressure for a given temperature and thereafter simply adjust pressure up or down in order to compensate for temperature changes. The converse is also true.

FIG. 3a illustrates a system for monitoring drop velocity directly and developing signals to control pump pressure in order to change the velocity of the drops, as required. FIG. 3a makes use of digital techniques. FIG. 3b is related to FIG. 3a using essentially the same sensor arrangement but developing analog signals to change pump pressure rather than digital signals that have to be converted to analog signals.

In FIG. 3a, a stream of drops 43 is emitted from nozzle 44 passing through a charge electrode 45. Gutter 46 is positioned for receiving drops in stream 43. Two sensors 48 and 49 are positioned a predetermined distance apart and in proximity to the path of travel of the drops in stream 43. The two sensors 48 and 49 feed respectively associated comparator circuits 50 and 51. The comparator circuits have reference potentials applied by lines 53 and 54, respectively. During testing of drop velocity, as when the nozzle 44 is at home position, or in between characters, gate circuit 56 is activated in a synchronous fashion by machine clock pulses on line 58. The comparator outputs are fed to gate 56 by lines 60 and 61 through interface connections 62 and 63, respectively.

In operation, a group of drops is emitted from nozzle 44, such as six (6) in number, or the like. The group of drops passes sensor 48 developing a voltage which ultimately activates gate 56 to gate counter circuit 65 to start a counting operation. When the sequence of drops passes sensor 49, another potential is developed that is also applied to gate 56 but that turns off counter 65 instead. Thus a number of count pulses is developed in counter 65 that is directly representative of the time required for passage of the drops from sensor 48 to sensor 49. The count status of counter 65 is applied to the digital-analog converter circuit 67 in order to derive a correction signal by line 68 that ordinarily would be applied to a pump control circuit similar to circuit 7 in FIG. 1 in order to vary pump pressure as required. As noted before, either the frequency or current drive of the pump can be changed in order to change pump pressure.

FIG. 3b is an analog approach utilizing various elements in FIG. 3a. The circuit of FIG. 3b is substituted for elements 56, 65, and 67 in FIG. 3a by appropriate connection of interface connectors 62a and 63a with connectors 62 and 63, respectively, in FIG. 3a. Outputs developed by sensors 48 and 49 in this case are applied to a ramp generator 70. Upon detection of a potential on line 60a, FIG. 3b, the ramp generator is activated. Ramp generator 70 develops a ramp signal at a known rate and range of voltage levels. Upon detection of another output on lines 61a, FIG. 3b, the output from ramp generator 70 is deactivated. The level attained is stored in the holding circuit 71 and applied by line 72 to vary pump pressure in a manner similar to that described before.

By using the foregoing techniques, the velocity of the ink stream 43 may be maintained constant. As a result, since the deflection sensitivity of stream 43 is proportion'al to l/( Vel), the deflection of stream 43 required during printing of information is also maintained in a tightly controlled manner. Using the servo techniques previously described, tolerances on other elements of the system, such as on the nozzle, temperature, etc. need not be maintained as tightly as would otherwise be required.

In FIG. 4, the actual deflection of a stream of drops is tested in order to determine velocity characteristics. Nozzle 75 emits a stream of drops 76 passing through charge electrode 77 and between deflection plates 78 and 79. During printing of information, the drops in stream 76 are directed to a record medium, not shown. When not required for printing, drops are directed to gutter 80. During testing of velocity of the stream, maximum deflection of drops is initiated by appropriate charging by charge electrode 77 and deflection by plates 78 and 79 in order that the drops reach the area of two proximity sensors 82 and 83 representing maximum deflection of the stream. As an example, a group of six drops can be used as before. Gutter 85 is positioned to receive drops directed between sensors 82 and 583. Potentials are developed by sensors 82 and 83 as the stream of drops passes by. It is assumed that a normal deflection for test purposes of the drops in stream 76 is between sensors 82 and 83. If drops pass close to sensor 83 representing an increase in velocity of the drops, an output is developed that is applied to amplifier circuit 90 and in turn to comparator circuit 91 for development of appropriate correction signal by line 92 to control pump pressure. In this case, since the velocity of the drops is somewhat high, the pump pressure is reduced. If drops pass in proximity to sensor 82, an output is developed that is applied to amplifier circuit 94 and again applied to comparator 91. In this case, the stream of drops is moving at a relatively lower velocity and the output signal by line 92 would be of an appropriate level to increase pump pressure.

FIG. 5 illustrates a highly efficient pump structure 100 that is useful in the various servo circuits previously described. Pump 100 includes a pump supporting structure 101 housing a number of elements. A flat spring member 102 is mounted for oscillatory movement in structure 101. Spring member 102 is driven by coil 104 that in turn is excited by an oscillator 106. Attached to member 102 is connecting rod 108 that in turn is connected to a bellows 110. Pump 100 further includes an input conduit 112 through which ink is supplied from an ink supply not shown. An output conduit 114 supplies ink to a nozzle, not shown, but that would be similar to those previously described. Control of ink passage and pumping is exerted by an input valve 115 and an output valve 116. The action of the pump is similar to that of a voice coil normally found in radio and television equipment, or the like. The metal diaphragm 102 and associated bellows 110 change the volume of the pump cavity 120. Valves 115 and 116 control the flow of ink in and out of the pump.

The pressure produced by pump 100 is related to the force imparted to bellows 1 by diaphragm 102 which in turn is related to the frequency and current conditions established in coil 104. With these characteristics, pump 100 is readily incorporated in the various servo circuits previously discussed and controllable as required insofar as maintaining a desired pressure range. This in turn, as mentioned, controls drop velocity.

In operation, as bellows 110 moves to the left in FIG. 5, flap 115 opens thereby drawing ink through conduit 112 into chamber 120. Valve 116 remains closed at this time. As bellows 110 moves to the right and expands, valve 115 remains closed and ink is forced through valve 116 and out of way by conduit 114 to the ink jet nozzle.

While the invention has been particularly shown and described with reference to several embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departure from the spirit and scope of the invention.

What is claimed is:

1. In an ink printing apparatus, a servo system for monitoring and maintaining parameters, affecting quality of printing, such as velocity of the jet, within predetermined ranges, which determines jet placement during printing of information, comprising:

jet forming means for forming and propelling an ink jet in a predetermined path of travel,

pump means interconnected with said jet forming means for maintaining a predetermined level of ink jet pressure in said jet forming means;

sensor means proximately positioned in relation to said ink jet for sensing a characteristic of said ink jet and for developing a signal representative of said characteristic;

comparator means for comparing said developed signal with a reference signal in order to further de velop a corrective signal; and

means for applying said corrective signal to said pump means in order to maintain pressure exerted by said pump means in said jet forming means within a predetermined range, thereby maintaining jet velocity, jet placement, and printing of information within a predetermined range of printing.

2. The apparatus of claim 1, further comprising:

pump control means interconnected with said pump means for directing corrective signals from said comparator means to said pump means; and

timing logic interconnected with said pump control means for controlling activation and deactivation of said pump control means.

3. The apparatus of claim 2 wherein said ink jet is directed to a medium for recording of information in the form of character intervals, each separated by an intercharacter interval; and further comprising:

recognition means interconnected with said logic means for recognizing said inter-character intervals and for activating said pump control means during said intercharacter intervals. I

4. The apparatus of claim 3 wherein said ink jet forming means and said medium are relatively moved from a home position to record information, and further comprising:

means in said recognition means for activating said pump control means while said apparatus is at home position.

5. The apparatus of claim 1, wherein said sensor means comprises:

a temperature sensor for monitoring temperature characteristics of said ink jet.

6. The apparatus of claim 1, wherein said sensor means comprises:

a pressure sensor for monitoring pressure characteristics of said ink jet.

7. The apparatus of claim 1, further comprising:

a temperature sensor and a pressure sensor incorporated in said sensor means and interconnected with said pump means for monitoring temperature and pressure, respectively; and

circuit means responsive to signals developed by said temperature and pressure sensors and interconnected with said pump means for controlling pump pressure in order to maintain said pump pressure within a predetermined range.

8. The apparatusof claim 1, further comprising:

a first proximity sensor and a second proximity sensor incorporated in said sensor means and connected for input to said comparator means, said proximity sensors being positioned a predetermined distance apart and adjacent the path of travel of said ink jet;

means for developing signals from said proximity sensors indicative of the passage of ink as it moves past said proximity sensors; and

said comparator means developing a corrective signal responsive to the signals derived from said first and second proximity sensors for application to said pump means in order to maintain pressure in said-pump means within a predetermined range.

9. The apparatus of claim 8, wherein said comparator means further comprises:

activatable gate means;

means interconnecting said proximity sensors as inputs to said gate means;

count means;

means interconnecting said gate means and said count means to initiate operation of said count means under control of said gate means during an activate mode of said gate means in order to develop digital count representations; and

means for activating said gate means and thereby said count means upon sensing passage of ink moving past said first proximity sensor and for deactivating saidgate means and said count means upon sensing passage of ink past said second proximity sensor.

10. The apparatus of claim 9, wherein said comparator means further comprises:

digital-analog converter means interconnected between said count means and said pump means for converting digital representations from said count means to an analog signal for application to said pump control means.

11. The apparatus of claim 8, wherein said comparator means further comprises:

a ramp generator circuit providing a ramp signal having predetermined slope and duration characteristics;

means interconnecting said proximity sensors as inputs to said ramp generator;

an analog holding circuit;

means for initiating operation of said ramp generator circuit upon sensing passage of ink by said first proximity sensor and for terminating operation of said ramp circuit upon sensing passage of ink by said second proximity sensor;

means interconnecting said ramp generator to said analog holding circuit in order to provide the ramp level attained by said ramp circuit to said analog holding circuit; and

means interconnecting said holding circuit to said pump means in order to correct pressure in said pump means.

12. The apparatus of claim 1 wherein said ink jet passes through a charge electrode and between deflection electrodes for charging and deflection within a predetermined deflection monitoring range, and further comprising:

means for applying a charging potential to said charging electrodes in order to deflect said ink jet into said monitoring range;

sensor means positioned adjacent the path of travel of said ink jet for developing signals from said ink jet during passage thereof past said sensor means indicative of velocity characteristics in said monitoring range, and

means for applying said velocity characteristics signals to said pump control means in order to correct pressure in said pump means.

13. The apparatus of claim 12, further comprising:

first and second sensor probes incorporated in said sensor means, said probes being positioned a predetermined distance apart and in proximity to said ink jet when said ink jet passes through said monitoring range; and

means interconnected with said probes and said pump means and responsive to signals developed by said probes for developing corrective signals for application to said pump means in order to increase or decrease pump pressure, as required, in order to maintain jet velocity within a predetermined range. 

1. In an ink printing apparatus, a servo system for monitoring and maintaining parameters, affecting quality of printing, such as velocity of the jet, within predetermined ranges, which determines jet placement during printing of information, comprising: jet forming means for forming and propelling an ink jet in a predetermined path of travel, pump means interconnected with said jet forming means for maintaining a predetermined level of ink jet pressure in said jet forming means; sensor means proximately positioned in relation to said ink jet for sensing a characteristic of said ink jet and for developing a signal representative of said characteristic; comparator means for comparing said developed signal with a reference signal in order to further develop a corrective signal; and means for applying said corrective signal to said pump means in order to maintain pressure exerted by said pump means in said jet forming means within a predetermined range, thereby maintaining jet velocity, jet placement, and printing of information within a predetermined range of printing.
 2. The apparatus of claim 1, further comprising: pump control means interconnected with said pump means for directing corrective signals from said comparator means to said pump means; and timing logic interconnected with said pump control means for controlling activation and deactivation of said pump control means.
 3. The apparatus of claim 2 wherein said ink jet is directed to a medium for recording of information in the form of character intervals, each separated by an inter-character interval; and further comprising: recognition means interconnected with said logic means for recognizing said inter-character intervals and for activating said pump control means during said intercharacter intervals.
 4. The apparatus of claim 3 wherein said ink jet forming means and said medium are relatively moved from a home position to record information, and further comprising: means in said recognition means for activating said pump control means while said apparatus is at home position.
 5. The apparatus of claim 1, wherein said sensor means comprises: a temperature sensor for monitoring temperature characteristics of said ink jet.
 6. The apparatus of claim 1, wherein said sensor means comprises: a pressure sensor for monitoring pressure characteristics of said ink jet.
 7. The apparatus of claim 1, further comprising: a temperature sensor and a pressure sensor incorporated in said sensor means and interconnected with said pump means for monitoring temperature and pressure, respectively; and circuit means responsive to signals developed by said temperature and pressure sensors and interconnected with said pump means for controlling pump pressure in order to maintain said pump pressure within a predetermined range.
 8. The apparatus of claim 1, further comprising: a first proximity sensor aNd a second proximity sensor incorporated in said sensor means and connected for input to said comparator means, said proximity sensors being positioned a predetermined distance apart and adjacent the path of travel of said ink jet; means for developing signals from said proximity sensors indicative of the passage of ink as it moves past said proximity sensors; and said comparator means developing a corrective signal responsive to the signals derived from said first and second proximity sensors for application to said pump means in order to maintain pressure in said pump means within a predetermined range.
 9. The apparatus of claim 8, wherein said comparator means further comprises: activatable gate means; means interconnecting said proximity sensors as inputs to said gate means; count means; means interconnecting said gate means and said count means to initiate operation of said count means under control of said gate means during an activate mode of said gate means in order to develop digital count representations; and means for activating said gate means and thereby said count means upon sensing passage of ink moving past said first proximity sensor and for deactivating said gate means and said count means upon sensing passage of ink past said second proximity sensor.
 10. The apparatus of claim 9, wherein said comparator means further comprises: digital-analog converter means interconnected between said count means and said pump means for converting digital representations from said count means to an analog signal for application to said pump control means.
 11. The apparatus of claim 8, wherein said comparator means further comprises: a ramp generator circuit providing a ramp signal having predetermined slope and duration characteristics; means interconnecting said proximity sensors as inputs to said ramp generator; an analog holding circuit; means for initiating operation of said ramp generator circuit upon sensing passage of ink by said first proximity sensor and for terminating operation of said ramp circuit upon sensing passage of ink by said second proximity sensor; means interconnecting said ramp generator to said analog holding circuit in order to provide the ramp level attained by said ramp circuit to said analog holding circuit; and means interconnecting said holding circuit to said pump means in order to correct pressure in said pump means.
 12. The apparatus of claim 1 wherein said ink jet passes through a charge electrode and between deflection electrodes for charging and deflection within a predetermined deflection monitoring range, and further comprising: means for applying a charging potential to said charging electrodes in order to deflect said ink jet into said monitoring range; sensor means positioned adjacent the path of travel of said ink jet for developing signals from said ink jet during passage thereof past said sensor means indicative of velocity characteristics in said monitoring range, and means for applying said velocity characteristics signals to said pump control means in order to correct pressure in said pump means.
 13. The apparatus of claim 12, further comprising: first and second sensor probes incorporated in said sensor means, said probes being positioned a predetermined distance apart and in proximity to said ink jet when said ink jet passes through said monitoring range; and means interconnected with said probes and said pump means and responsive to signals developed by said probes for developing corrective signals for application to said pump means in order to increase or decrease pump pressure, as required, in order to maintain jet velocity within a predetermined range. 